This invention relates to rotary displacement pumps, and more particularly, to a rotary displacement pump having a separable member that controls transitional regions and thereby the fluid flowpath.
Many types of rotary displacement pumps have been used for various purposes. A description of the design and operation of rotary displacement pumps can be found in the McGRAW-HILL book titled "Pump Handbook" edited by Karassik, Krutzsch, Fraser, and Messina, on pages 3-70 through 3-99, section 3.4 titled "Rotary Pumps" by C. W. Little, Jr., which is hereby incorporated by reference. Furthermore, the following US-A Patents can be referenced for more teachings on the design and uses of rotary pumps: U.S. Pat. Nos. 5,160,252 to Edwards; 5,144,802 to Ruzie; 5,096,390 to Sevrain et al.; 4,770,616 to Kahrs; 4,229,147 to Linder et al.; and, 3,873,231 to Callahan.
In general, a rotary displacement pump can be loosely defined as one in which the main pumping action is caused by relative movement between rotating elements and stationary elements. One popular type of rotary pump is the vane and rotor pump. Although this type of pump shall be described herein, it should be noted that the present invention is applicable to other designs and is not limited to rigid vane rotary pumps. For instance, other applicable pumps can include an external gear pump, an internal gear pump, a screw and wheel pump, a multiple rotor screw pump, an external circumferential piston pump, an internal circumferential piston pump, various lobe pumps, a single screw pump, external vane-in-body pump, a flexible vane pump, a flexible liner pump, and a flexible tube pump.
Transitional inlet and outlet regions into the fluid flowpath of a pump are critically important in almost all pump applications as described in the "Pump Handbook." These transitional regions are often recessed ports which change the internal geometry of the flowpath within the pump at the areas where fluid enters the flowpath and exits the flowpath. Properly designed transitional regions are important primarily for two reasons. First, without proper port design, unfavorable flow inlet and outlet conditions will develop resulting in inlet and outlet losses that lower the efficiency of the pump. Secondly, without properly transitional regions, there will be sharp corners in the flow path geometry causing rapid flow direction changes. This greatly increases the likelihood of cavitation especially at higher pump speeds or when the fluid medium is of low viscosity. Cavitation is vapor formation in the form of very tiny bubbles at any area in the flow path where the local pressure approaches or drops below the vapor pressure of the fluid medium.
Cavitation adversely affects pump performance in three ways. Since this type of pump is a fixed displacement device, if, for example, only half of the actual fluid volume swept from the inlet chamber transitional region is vapor, then only half the normal capacity liquid volume is available at the outlet transitional region chamber, and the pump capacity is reduced accordingly. Secondly, physical damage to internal pump components and surfaces is caused by the collapse of cavitation vapor bubbles. This is known as cavitation erosion or pitting. Finally, cavitation often results in noisy and rough operation caused by the formation and collapse of vapor bubbles. All three have a negative effect on pump efficiency and/or component life.
Another desirable attribute which can be used in designing a rotary displacement pump is that of a substantial colinear alignment of the inlet and outlet transitional regions and corresponding inlet and outlet fittings. Colinear alignment of the inlet and outlet fittings makes plumbing connections in many applications greatly simplified. A straight section of fluid plumbing, for instance, can be broken and the pump inserted directly in the line with no further modification. Not only does the straight through flow offer simple system connections, it also has additional system benefits. For instance, some systems rely on rapid and efficient drain back of fluid through the system, and specifically the pump. Any configuration other than the straight through flow configuration would trap significantly more fluid after drain back, therefore, the colinear inlet and outlet fitting configuration is very important to pump performance in many applications.
As noted above, transitional region design and system inlet and outlet transitional regions aligned substantially in a colinear manner are two features that are critical to pump performance, but can be difficult and costly to manufacture. Specifically, when the straight through inlet and outlet fitting alignment is combined with the necessity for transitional regions provided by recessed inlet and outlet ports, severe tooling difficulties are encountered. These difficulties are the reason that pumps of this configuration are either made from processes such as metal casting, made without recessed port designs, or made with inlet and outlet fittings which are not colinear. As will be seen with reference to FIGS. 1, 2a, and 2b, each of these possibilities are generally inferior to the proposed invention.
For the purpose of illustration, FIG. 1 shows the components and critical flow path elements of a standard rotary vane pump 10 of the prior art. Rotary pump 10 includes a body 11, transitional inlet region 12, transitional outlet region 14, rotor 16, vanes 18, inlet fitting 20 outlet fitting 22 and internal flow area 24. Rotor 16 is disposed in an eccentric relationship to the internal casing of body 11.
As rotor 16 and vanes 18 rotate inside the stationary body 11, volumes of fluid are transferred through a channel and inlet port in inlet fitting 20 into transitional inlet region 12. Propelled by rotor 16, the fluid then flows through internal flow area 24 to transitional outlet region 14 then out of pump 10 through an outlet port and channel in outlet fitting 22. Note that the areas defining transitional inlet region 12 and transitional outlet region 14 are recesses in the substantially circular interior of pump 10. As discussed above, the shapes of these transitional regions are critical to pump performance. Also note, that the flow direction at inlet fitting 20 is substantially the same as at the outlet fitting 22 making the pump colinear.
Pump 10 of FIG. 1 can be adequate for some applications, however, it is inflexible in design and expensive to manufacture. The pump as shown requires a casting method to be manufacturable, and has the typical configuration of a body housing all the pumping components along with a separable cover (not shown). The transitional regions of this device are designed to a meet certain flowpath specifications which may be determined by the fluid it will be pumping or the velocity the fluid will be traveling through the pump. A redesign would be required to change the flow path specifications to meet requirements for a fluid with different characteristics or for a different fluid velocity. In addition, manufacturing this device using a metal casting method could be very expensive. Clearly, a plastic injection molding process for manufacturing the device would be preferred due to the diverse range of materials to choose from and the low cost of manufacturing using injection molding. However, due to the transitional regions being integral with the inlet and outlet ports, manufacturing this device using injection molding is difficult. Furthermore, a design which could be easily changed to permit the use of a variety of fluids and fluid velocities would be more desirable.
FIGS. 2a and 2b show two popular examples of changing pump geometry to enable manufacturing by injection molding and therefore less expensive pump designs than that of FIG. 1. Note that both pumps include a main body housing the pumping elements with a cover (not shown). Pump 26 of FIG. 2a has a colinear design which is desirable in many applications. However, pump 26 does not provide for varied design in either transitional inlet region 28 or transitional outlet region 30. The transitional regions of pump 26 are simply ports, with no specific design or shape to increase pump efficiency. Therefore, pump 26 is only a marginal design because the transitional regions cannot be shaped to enhance pump efficiency.
Pump 34 of FIG. 2b is another example of a pump which can be manufactured using injection molding. Pump 34 of FIG. 2b has a limited ability to change the design of transitional inlet region 36 and transitional outlet region 38. However, the inlet and outlet fittings to the pump are not colinear making this pump inappropriate for certain applications.
The performance losses and hazards of improperly designed transitional regions are well documented. The system installation and system drain back benefits of having colinear inlet and outlet fittings can not be obtained by any other configuration. Therefore, it is recognized that a design for a rotary displacement pump allowing flexible design of transitional regions which is inexpensive to manufacture would be desirable. In addition, a pump having transitional regions on a member which is separable and easily modified for change in fluid characteristics or fluid velocities would also be desirable. Furthermore, an additional advantage would be realized if the pump could be manufactured using the plastic injection molding process.